The present invention relates to an electric power converting system using switching elements.
Measuring of currents at various portions in an electric power converting system has been widely performed.
For example, a measured result of current is used for judging whether or not an over current is flowing in an element such as an IGBT (insulated gate bipolar transistor) or a MOSFET due to occurrence of a short-circuit in the element. Because an element such as an IGBT or the like has the current saturation characteristic that an output current is limited by the gate voltage, the current can be turned off without breakdown if the current is turned off within several tens .mu.s in the event of occurrence of a short-circuit status in which a voltage of a power source is directly applied to the element. Therefore, it is becoming more common that an electric power converting system comprises some means capable of judging occurrence of the short-circuit status, and contains a short-circuit protective unit for self-turning off the element when it is judged that the short-circuit status occurs in the element.
The method of judging occurrence of a short-circuit status commonly used is that current flowing in an element is measured and it is judged occurrence of the short-circuit status when the measured current exceeds a reference value. Various methods of detecting the current are known. For example, a technology disclosed in Japanese Patent Application Laid-Open No. 5-267580. In the method, an auxiliary element dedicated to measuring current flowing through an element is connected to the element in parallel, and the current is measured by a voltage generated in a resistor connected to the auxiliary element in series. This method has an advantage in that a large current can be measured because a part of current flowing through the main element flows through the auxiliary element. Therein, the auxiliary element is usually included inside a chip together with the main element.
There are the other methods of measuring current flowing through an element, one method is that a voltage is measured by directly connecting a resistor to a main element in series, not using the auxiliary element, and another method is that a voltage is measured by arranging a current transformer in a wire of a main element. These methods have an advantage in that the current can be read with a high accuracy because the current flowing through the element is directly read.
Recent electric power converting systems are not only equipped with the protective unit described above, but also being improved in performance of PWM control. As one of the examples, there is vector control of an induction motor. Although the induction motor is widely used because it is simple and tough in structure of the rotor and low in price, it is necessary to use a vector control technology in order to stably drive the induction motor.
Similarly to the protection of short-circuit, the vector control also requires to measure current with high accuracy. In the vector control, it is necessary to read current flowing Inti the motor as the control information, and the current is usually measured by a voltage generated between terminals of a resistor inserted in a wire between an inverter and the motor or by a current detector such as a current transformed attached to the wire.
As described above, the electric power converting system requires to measure currents in the switching elements and the motor with high accuracy, and the currents are measured using the auxiliary element, the resistor or the current transformer.
However, the conventional methods of measuring the currents have the following problems.
In the method of measuring current using the auxiliary element, the series circuit of the auxiliary element and the resistor is connected to the main element in parallel, and a current is read from a voltage generated in the resistor. Because the voltage applied between the terminals of the main element is divided by the auxiliary element and the resistor in the side of the auxiliary element, voltage applied between the terminals is different between the main element and the auxiliary element. Further, since the voltage generated in the resistor is increased in proportional to the current value flowing through the resistor, the ratio of the voltage applied to the main element to the voltage applied to the auxiliary element varies depending on the current value. Therefore, the method of using the auxiliary element is difficult to measure the current with high accuracy because the ratio of the current flowing through the main element to the current flowing through the auxiliary element varies depending on the current value.
On the other hand, the method of measuring the current by connecting the resistor to the main element in series can measure the current with high accuracy because all the current flowing through the main circuit flows through the resistor, but a large loss occurs in the resistor. Therefore, it is difficult to measure a current above several tens Amperes.
Furthermore, the method of measuring the current using the current transformer can measure the current with high accuracy, but it is difficult to make the system small in size and low in cost because the current transformer is large in size and high in price.